Overscan is a problem of CRT tvs, digital tvs (hdtv, lcd ..) don't really have this issue. You might have noticed that all televisions have different sizes. Small 37cm/14" televisions appear to be 4:3 while big televisions are wider they are closer to 5:3 than to 4:3 while the image covers the whole screen in both cases. What happens is that televisions receive a 'too big' picture of which they don't show everything. This is the origin of overscanning.

When you connect a dvd player to your tv, again the tv receives a big picture which isn't fully shown. The overscan ratio of the dvd player has been chosen to such a value that allmost nobody sees black borders.

A computer screen works with pixels whereas a television doesn't work with pixels. A tvout encoder (or part of the gpu) converts the image to something the tv understands. When you output your desktop to your tv, you don't want to lose any part of your desktop while you don't want black borders either. Nvidia had to choose a value for overscanning which worked reasonably for most users. The overscan options can tweak this a bit but it is allways a tradeoff between black borders and the whole image.

For movie playback I would just live with the fact that you lose a small part of the image as that is also what happens for normal media players and television.